At present there are two types of high-resolution electro-optical screens: LCD (Liquid Crystal Display) screens—see in particular the document U.S. Pat. No. 6,052,162—and OLED (Organic Light Emissive Diodes) screens—see in particular the document WO-2007/085554. It can be noted that, unlike OLED screens, LCD screens are not emissive screens.
A problem common to these two types of screen is the difficulty in combining high resolutions (many image points or pixels) with a small screen size (in order to reduce the cost). It is therefore sought to produce increasingly smaller pixels, present-day pixels having a size of about 5 μm, and the ability to produce pixels whose size is 1 μm, or even 500 nm, is sought.
LCD or OLED screens are generally produced on an active matrix, which is in practice made of monocrystalline silicon, in which an integrated addressing circuit, based on transistors, is formed. It is also possible to produce this addressing circuit based on TFT (Thin Film Transistor) transistors. In this case, these transistors are produced in films of amorphous or polycrystalline silicon deposited on a glass substrate. Whatever the technology may be (LCD or OLED), the active matrix used comprises, along its surface in contact with electro-optical parts of the screen, electrodes defining the pixels of the screen: in order to take maximum advantage of the area of the active matrix, these pixel electrodes occupy the major part of this area.
Upon this active matrix are added electro-optical zones respectively situated opposite each pixel electrode (and under a counter-electrode). In the case of an LCD screen, the electro-optical zones are constituted by a liquid crystal whose transparency varies according to the polarization applied between the two electrodes. In the case of an OLED screen, the electro-optical zones, which are emissive, are constituted by a central portion of a light emitting diode which emits, or does not emit, radiation according to the state of excitation imposed by the transistors of the subjacent active matrix.
Respective types of constraints correspond to each type of screen.
In the case of LCD screens (in practice it is more precisely a matter of screens of the reflective LCOS (reflective “Liquid Crystal On Silicon” on integrated circuit) type or of the transmissive LCD on glass type (notably marketed by the Kopin company), one of the main problems for the small pixels is controlling the state of the liquid crystal between adjacent pixels. In fact, the field lines created at the level of a pixel can affect the field lines of the adjacent pixels, generating interference at the level of these pixels. This prevents the production of screens having pixels with a pitch of less than ˜5 μm.
In the case of OLED screens (such as those of the companies Emagin, MED, or MicroOLED, in particular), the OLED diodes generate white light which is then colored, if necessary, by colored (typically red, green and blue) filters. In this case there is a matrix of pixel electrodes surmounted by a white OLED deposit and cooperating with a system of colored filters which is either deposited on the matrix (the case of the “Emagin” products) or produced on a glass plate and assembled with the active matrix (the case of the “MED” or “MicroOled” companies).
A first problem arises regarding the deposit of the layers constituting the OLED diodes on the active matrix. In fact, the pixel electrodes in practice constitute steps with respect to the surface of the active matrix; the fact that the emissive part is formed of layers causes these steps to remain in this emissive part; in particular the conductive layer forming the counter-electrode opposite the pixel electrodes (in practice it is a matter of the layer by which the emissive part terminates) thus comprises steps, which constitute risks of short-circuit (at the location of the level changes, the layers risk being interrupted and causing inadvertent contacts). In order to reduce this risk, it is necessary to apply, locally between the pixels, an insulating resin and thus to provide transition zones, the consequence of which is a loss of aperture of the active surface because of the design rules (this resin neutralizes the periphery of the pixel electrodes). The percentage of active surface (sometimes denoted by OAR) can drop to 25% for the case of 5 μm pixel pitches. It is understood that this problem hampers the production of high-resolution (small pixel) OLED screens by causing the percentage of emissive surface to drop.
A second problem concerns with the use of colored filters. In fact, whether these are deposited directly on the active matrix, or assembled with the active matrix, there is a loss of aperture related to the design rules in the first case, or related to the alignment precision in the second case. This results in an additional limitation of the active surface aperture.
A third problem concerns the extreme fragility of the OLED layers, in particular with regard to impurities such as water for example. Specific encapsulations are used but aging problems nevertheless persist with this type of screen. In addition to the above mentioned difficulties is the problem that, whatever technique is used (LCD or OLED), the possible presence of particles at the interface between the active matrix and the emissive layers generates risks of horizontal short-circuit between adjacent pixels (which is a crippling risk which considerably hampers the reduction of inter-pixel pitches and therefore the resolution).